1. Field of the Invention
The present invention relates to a fuel cell comprising a membrane electrode assembly and first and second separators sandwiching the membrane electrode assembly. The membrane electrode assembly includes a first electrode and a second electrode, and an electrolyte membrane interposed between the first electrode and the second electrode. The surface area of the second electrode is larger than the surface area of the first electrode.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. In use, a predetermined number of power generation cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. A gas chiefly containing oxygen or air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In the power generation cell, in order to prevent the leakage of the fuel gas and the oxygen-containing gas by providing hermetical fluid passages, various sealing structures are adopted. For example, Japanese Laid-Open Patent Publication No. 2002-25587 discloses a fuel cell in which it is possible to improve the sealing performance between a membrane electrode assembly and separators.
In the fuel cell, as shown in FIG. 8, a power generation cell is formed by sandwiching a membrane electrode assembly 1 between first and second separators 2a, 2b. The membrane electrode assembly 1 includes an anode 4a, a cathode 4b, and a solid polymer electrolyte membrane 3 interposed between the anode 4a and the cathode 4b. The surface area of the anode 4a is larger than the surface area of the cathode 4b. 
A first seal 5a is attached to an inner surface of the second separator 2b. The first seal 5a is provided around the cathode 4b, and tightly contacts the solid polymer electrolyte membrane 3. Further, a second seal 5b is provided between the first and second separators 2a, 2b. The second seal 5b is provided around the first seal 5a. 
In the conventional technique, a reactant gas leaks through a space formed between the first and second seals 5a, 5b. The reactant gas may not flow through a reactant gas flow field (not shown), and may pass through the region around the anode 4a, and a so-called short cut may occur undesirably. In this case, the reactant gas is not reliably supplied to the electrode reactant surface. Thus, the desired power generation performance cannot be achieved.